1. Field of the Invention
The present invention relates to an exposure device and an image forming apparatus using the same, and more particularly, to an exposure device and an image forming apparatus using the exposure device in which light emitting elements are aligned in an array configuration.
2. Description of the Related Art
With the recent trend toward a smaller and cheaper image forming apparatus such as a facsimile machine or a printer, there have been many attempts to make smaller and cheaper components of the image forming apparatus.
In an image forming apparatus employing an electro-photographic process, a photosensitive member is exposed to light by irradiating light modulated with image data onto the photosensitive member, and a toner adhering to the photosensitive member by the electrostatic force is transferred onto a printing medium such as a recording paper, thereby forming an image on the printing medium. As a method of controlling the light irradiated onto the photosensitive member, there have been known a method of introducing light emitted from a semiconductor laser onto the photosensitive member using a rotating polygonal mirror, and a method of achieving the same effect by using an exposure device known as an optical head having a plurality of very-small light sources. The optical head includes a light source and a circuit for controlling driving of the light source, and a light emitting diode is commonly used as the light source.
As a means for downsizing the light source while reducing the manufacturing cost, there has been proposed an optical head using an EL (electro-luminescence) element as its light source.
Examples of the optical head using an organic EL element as the light source are disclosed in Patent Documents 1 and 2. Patent Document 1 describes the configuration of the optical head with a focus on light emitting and detecting elements, and Patent Document 2 describes the configuration with a focus on pixels. The light emitting units disclosed in both Patent Documents have substantially the same configuration and are configured as a stacked layer structure including a light emitting layer constituting the organic EL element, a thin-film transistor serving as the light detecting element for use in light intensity correction and as a circuit for controlling the driving of the light emitting layer, and the like.
The light detecting elements disclosed in both Patent Documents have a light receiving area smaller than the light emitting area of the light emitting layer to prevent the light output from the bottom surface of the stacked layer structure from being blocked at the light receiving area.
Patent Document 1: JP-A-2002-144634
Patent Document 2: JP-A-2002-178560
FIG. 33 is a cross-sectional diagram of a configuration of an optical head, and more particularly, of a peripheral configuration of an light emitting element provided in the optical head, in accordance with the related art.
As shown in FIG. 33, a light emitting element (an EL element 110) serving as a light source of an optical head is configured as a stacked layer structure including a plurality of layers made of various kinds of materials. In the optical head, a base coating layer 101 is prepared on a glass substrate 100, the EL element 110 and its driving circuit serving as the light source and the driving circuit are formed thereon, and a light detecting element 120 constituted, for example, by a thin-film transistor is formed on a portion of the base coating layer 101.
Next, an interlayer insulating film 103 made, for example, of a silicon oxide film is formed on the stacked layer structure. In addition, an anode 111, a light emitting layer 112, and a cathode 113 are formed thereon. In this state, when an electric voltage is applied between the anode 111 and the cathode 113, electric potential difference is provided across the light emitting layer 112 and thus the light emitting layer 112 is caused to emit light.
As shown in FIG. 33, the optical head known in the art is provided with a light detecting element 120 which has a light receiving area Ar smaller than a light emitting area ALE of the light emitting layer to prevent the light output from the bottom surface of the stacked layer structure from being blocked at the light receiving area.
Therefore, at this moment, a bump area resulting from the light detecting element 120 is present on the surface of the stacked layer structure. Next, an interlayer insulating film 103 made of a insulating film such as an silicon oxide film is formed on the stacked layer structure. However, it is difficult to form the interlayer insulating film 103 with a uniform thickness due to the above-described bump area resulting from the light detecting element 120. Thus, the surface of the interlayer insulating film 103 is formed into a convex curved surface corresponding to the shape of the light detecting element 120. Therefore, the surfaces of layers formed on the interlayer insulating film 103 are also formed into the convex curved surface corresponding to the light detecting element 120, and the light emitting layer 112 may have a small thickness at areas such as the convex portion or its edge portions. Accordingly, the thickness of the light emitting layer 112 is not maintained at a constant level over a light exiting area ALE. In addition, the brightness of the light emitting layer 112 at the surfaces of the small-thickness areas may be greater than that of other surfaces. Accordingly, the intensity distribution (i.e., in-plane distribution) of light emitted from a single EL element 110 may become irregular.
The thin-film transistor constituting the light detecting element 120 faces the anode 111 of the EL element 110 via the interlayer insulating film 103. As a result of many experiments, it has been found that the light detecting element 120 is greatly influenced by the electric potential of the anode 111 and functions as a thin-film transistor having, as its gate electrode, the anode 111 of the EL element 110.
In the optical head known in the art, it is not considered that the light detection precision of the light detecting element 120 is greatly influenced by the electric potential of the anode 111, and more particularly, it does not suggest the layout and size of the light detecting element 120 with respect to the anode 111 of the EL element 110.